WOBURN — Debra Stone-Sochat fought through tears Wednesday as she confronted Thomas Mortimer IV, the man who killed her mother, her sister, and her sister’s two small children in a horrific act of domestic violence.

Stone-Sochat delivered a victim impact statement in Middlesex Superior Court after her former brother-in-law, Mortimer, pleaded guilty to four counts of first-degree murder for killing his family inside their home on Windsong Lane in Winchester in June 2010.

Stone-Sochat, who was in contact with her sister, mother, and the children on a nearly daily basis until the day they were killed, said she had struggled to regain joy in her life since the murders.

“My family has suffered an enormous tragedy,’’ Stone-Sochat said. “My life, my family’s life, my children’s life, have been so affected. It is difficult to find joy when the sadness is so overwhelming. My children wonder why they can’t have play dates with Charlotte and Finn. My mom and my sister were my best friends.’’

Mortimer, who pleaded guilty before Superior Court Judge S. Jane Haggerty, was impassive when the prosecution described the violence he had perpetrated on his four victims, a gruesome recitation that included the number of stab wounds suffered by each.

When Stone-Sochat finished reading from a prepared statement she held in her shaking hands, she raised her head up and addressed the victims: “Love you. And miss you.’’

She never addressed Mortimer directly.

Defense lawyer Denise Regan read aloud a letter written by Mortimer’s parents. In it, Mortimer’s parents said that they missed their grandchildren, and also indirectly faulted themselves for not being aware of how depressed their son had become.

“We know that our son, Thomas, has done something horrible,’’ the parents wrote, adding that they still consider him one of the “nicest persons” they have known.

“We didn’t know he was so depressed,” the parents said. Regan told the court that Mortimer had a documented history of treatment for depression. She also said Mortimer had a “deep and profound remorse” for the murders of his family.

Haggerty accepted Mortimer’s guilty plea and sentenced him to mandatory life in prison without the possibility of parole. Before Mortimer was led away in handcuffs and shackles, Haggerty addressed him and credited him for choosing to plead guilty and not seek a trial.

“If redemption is even possible for you in the face of these atrocities, you have taken the first step in that direction,” the judge said. “You have spared the family, friends, your parents, first responders, the community, jurors that would have been empaneled the retelling and reliving of the unimaginable and gruesome details of the murders of your family.”

Mortimer did not speak in court, other than to answer routine legal questions. But when Winchester police entered the blood-soaked home, they found confessions Mortimer had written on a computer and printed out.

“What I have done is extremely selfish and cowardly,” the former sales executive wrote in one confession previously made public. He described cutting his children’s throats and stabbing his wife and mother-in-law.

He wrote that he exploded into a homicidal rage when he and his wife quarreled over a $2,499 check he wrote to the IRS that bounced.

“I took the easy way out. . . . I am ashamed, frightened, relieved, surprised that I murdered my family, disgusted with myself,” he wrote.

Mortimer wrote that he believed his two children were better off dead than to grow up as children from a divorced home. And in one of the most chilling sections of his confession, one that his lawyer tried to keep from being disclosed, Mortimer said his son had witnessed portions of the murders before he, too, was killed.

“I expecially [sic] sorry to Finn that he had to witness these horrid acts,” Mortimer wrote. “It was not supposed to be this way.”

The murders were discovered June 16, 2010, when Stone-Sochat went to check on her sister and mother, whom she had not heard from in two days. Stone-Sochat and a neighbor first tried to force their way in, but then called authorities, who found the bodies.

After the plea, Middlesex District Attorney Gerard T. Leone Jr. said Mortimer should not be given any credit for choosing to plead guilty. Mortimer’s decision, the prosecutor said, was a killer’s way of avoiding a public recounting of the harm he caused.

“This is a selfish, myopic defendant who in all likelihood did not want a public forum for his heinous, unspeakable acts,’’ Leone told reporters. He said Mortimer committed the “ultimate act of domestic violence.’’

Winchester Police Chief Kenneth C. Albertelli said after the hearing that the murders affected the entire community.

“It was very disturbing, certainly for the family, and I hope in some way they get some closure from this,” Albertelli said. “As much as it’s nice to see it closed out, I think the effects are going to linger for a while for a lot of people.”

If Mortimer had gone to trial and been convicted of even one count of first-degree murder, the sentence would have been mandatory life in prison without parole. But he would have had the right to have his trial automatically reviewed by the Supreme Judicial Court, which could overturn such a verdict.

By entering the plea, Mortimer surrendered his right to appeal. Pleas on first-degree murder are rare, Leone said, adding that the evidence against Mortimer was overwhelming.

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